The ability to manage user mobility without service interruption is a fundamental requirement in cellular networks. Particularly sensitive is the so-called handover or handoff procedure in which a user's communication link(s) is transferred from one (or more) base station(s) to one (or more) other base station(s) in the middle of an active session. The purpose of the handover procedure is to preserve ongoing calls or sessions, when moving from one cell to another.
The handover process can be “soft”, in which case an active set of multiple base stations maintains simultaneous connections with a given user and base stations are added and removed from the active set as radio conditions change, or “hard”, in which case a single serving base station passes on a connection in its entirety to another serving base station.
The decision whether to perform handover or active set update is usually made by a network node such as the base station controller in GSM, or the radio network controller in WCDMA. As a basis for this decision, the node receives information about the radio link quality from the base stations and mobile units under its control. During a call, the mobile unit sends measurement results to the base station or base stations with which it is communicating, either periodically, or when requested to do so by the network, or whenever some other pre-defined criterion is fulfilled. The measurement results typically contain measurements of the radio signal strength and quality of the downlink (from the base station(s) to the mobile unit) of the call, as well as the signal strengths of a number of neighboring base stations, e.g. six neighboring base stations in GSM. The serving base station or base stations measure the uplink (from the mobile unit to the base station(s)) radio signal strength and quality of the call and forward these measurement results, together with those from the mobile unit, in a measurement report to the aforementioned node that is responsible for handover and active set update decisions. From the information in the measurement reports, the node is then able to decide whether a handover to another cell or an active set update is needed.
A variation on the abovementioned procedure is that the mobile unit itself initiates the handover process based on its own measurement results. This is an option, for example, in GPRS systems.
Irrespective of which node actually makes the decision, handover signaling is necessarily performed at or near the cell borders, far from the serving base station(s) in relatively poor radio conditions. Picking the optimal time to initiate handover or active set update can be difficult in practice due to factors such as cell plan irregularities, neighbor cell measurement constraints, measurement filtering, hysteresis values, and channel management timers. As explained in more detail below, sub-optimal handover behavior can have a strong negative impact on service quality and overall system performance, making it essential that the handover procedure be robust to poor radio conditions.
State-of-the-art cellular networks typically employ a very tight reuse of radio resources to maximize spectral efficiency and simplify network planning. An exemplary method in GSM is Fractional Load Planning (FLP) in which frequencies are reused for traffic in each cell, so-called 1-reuse. Fewer transceivers than there are available frequencies are typically installed in each cell in order to guarantee that only a fraction of these frequencies are in use at any given time irrespective of the amount of traffic, hence the term “fractional loading”. Users in an FLP network perform frequency hopping between all the allocated frequencies, but in different patterns in different cells to create strong but sporadic interference. Any bit errors occurring during the times of interference can generally be corrected with the help of channel coding and interleaving, and the resulting spectral efficiency clearly exceeds that of traditionally planned networks with a sparse frequency reuse.
In traditional sparse reuse networks, system performance is relatively insensitive to users being connected to sub-optimal base stations because the interfering cells are typically far away from both the serving cell and its neighboring cells. Mobile units drifting well into a neighboring cell before performing handover do not generally cause a significant increase in interference and the demands on the handover process are therefore lower. In state-of-the-art tight reuse networks, however, interfering cells are close by and being connected to the most appropriate base station is essential if the high spectral efficiency potential is to be achieved. In many cases, the serving cell and its neighbors will be strong interferers and drifting well into a neighboring cell can dramatically increase interference levels, thereby disrupting user services and reducing system capacity. Battery lifetimes are also negatively affected.
Soft handover solutions are generally more robust than those using hard handover, but they involve higher complexity and cost in infrastructure and terminals, and they demand more transmission and air interface resources. In many practical situations, the introduction of soft handover is unfeasible.
Hard handover is easier and cheaper to implement and it utilizes fewer resources, but achieving the requisite robustness can be difficult, particularly in state-of-the-art networks, for the reasons outlined above.